1. Field of the Invention
The present invention relates to a monomode light waveguide, an optical fiber, made of quartz glass and to a method for producing it.
2. Description of Related Art
Light waveguides for the transmission of optical signals are composed of a light carrying core region and a cladding region surrounding this core region, with the core region having a slightly higher index of optical refraction than the cladding region. The light to be transmitted is essentially carried in the core region and only a relatively small amount of the light extends into the cladding region immediately surrounding the core region.
A distinction is made in light waveguides for optical communications transmission between multimode and monomode light waveguides. Multimode light waveguides, particularly the so-called gradient index fibers, have a typical core diameter of 50 .mu.m with a typical outer diameter of the cladding region of 125 .mu.m. Monomode light waveguides having the same outer diameter of the cladding region have a typical core diameter of 8 .mu.m to 10 .mu.m. Monomode light waveguides permit the realization of very much higher transmission bandwidths than multimode light waveguides. Therefore, monomode light waveguides are particularly well suited for long-distance transmission paths on which the repeaters are spaced far apart and also for optical broadband networks operating over short-distances (local networks), particularly if such monomode light waveguides can be manufactured more economically than the multimode type.
At present, quartz glass light waveguides are used almost exclusively in the field of optical communications transmission. The total dispersion of monomode fibers disappears in these waveguides at a wavelength around 1.3 .mu.m; at the same time, optical transmission losses under 0.5 dB/km are possible. To realize such low losses, the OH.sup.- ion concentration in the light carrying core region and in the adjacent cladding region must be no more than about 100 ppb, since at light wavelengths of 1.24 .mu.m and 1.39 .mu.m OH vibration bands in the quartz glass produce heavy optical transmission losses. Overlapping of these absorption bands may increase the optical losses over the entire spectral range extending from about 1.2 .mu.m to 1.55 .mu.m. The required low OH.sup.- ion concentration can be realized in quartz glass only with considerable technological expenditures and therefore is not cost effective.
The manufacture of light waveguides made of quartz glass is presently based almost exclusively on chemical gas phase precipitation of quartz glass. Various processes, e.g. the MCVD (modified chemical vapor deposition), the PCVD (plasma chemical vapor deposition), the OVD (outside vapor deposition) and the VAD (vapor axial deposition) process, have been develoed for this purpose. For example, in the MCVD process, highly pure quartz glass is precipitated from the gas phase on the inner walls of a substrate tube of quartz glass.
To produce a light waveguide from a preform, a suitably doped, precipitated quartz glass layers forms the core region and an inner cladding region, while the substrate tube forms the outer cladding region. This manufacturing process is particularly suited for the production of monomide light waveguides (monomode fibers) since in this process the volume of the core region is relatively small compared to the volume of the total fiber. Therefore it is necessary to precipitate relatively little highly pure and thus expensive quartz glass on a substrate tube made essentially of a much less expensive (quartz) material.
Since with monomode light waveguides part of the carried light extends into the cladding region, an inner jacket region likewise of highly pure quartz material must be provided immediately around the core region. If 2a is the diameter of the core region, this requires a diameter of about 4a. If, however, OH.sup.- ion are present in the substrate tube made of the less expensive quartz glass, these may diffuse into the inner cladding region during the production of the preform and during drawing of the fibers and may cause a considerable increase in attenuation. The diffusion of OH.sup.- ions was examined, for example, in a paper published in the periodical entitled "Japanese Journal of Applied Physics", Volume 17, No. 11, November, 1978, at pages 1975-1981. There it is stated that the ratio of the cladding radius to the core radius must be at least about five to one to realize an OH absorption of less than 20 dB/km at a light wavelength of 1.39 .mu.m.